In the formation of integrated circuits using bipolar transistors, the use of subcollectors for purposes of reducing series collector resistance has become commonplace. Conventional subcollectors consist of buried heavily doped regions underlying a more lightly doped collector region of the same conductivity type as the subcollector The collector region may be formed by epitaxial silicon growth over the buried region. The intrinsic base region is then formed by implant and diffusion into the epitaxial layer over above the subcollector, and the emitter is formed by diffusion into the intrinsic base region. An extrinsic base region, consisting of a heavily doped region within the intrinsic base region, and of the same conductivity type, is often used to reduce the series base resistance of the device.
Of course, for the subcollector to serve as the collector of the bipolar transistor, contact must be made thereto. This typically is provided by making a surface contact to the more lightly doped epitaxial collector region. However, the contact to the epitaxial layer introduces series collector resistance between the surface contact and the subcollector, such series collector resistance being in addition to the series resistance of the collector region between the intrinsic base and the subcollector. A less resistant contact may be made by using a deep diffusion from the surface of the structure, reaching fully to the subcollector region. Such a contact requires a long drive-in diffusion for the dopant to diffuse from its implanted or deposited location to fully extend from the surface to the subcollector. This long diffusion also results in significant lateral diffusion of the dopant, which not only increases the surface area required for the implementation of the transistor, but also reduces the ability to scale the transistor to smaller geometries.
Furthermore, for conventional bipolar transistors which have a subcollector region which fully extends under both the intrinsic and extrinsic base regions, the base-to-collector capacitance is high, due to the proximity of the heavily doped extrinsic base region to the heavily doped subcollector of opposite conductivity type lying directly thereunder.
As noted above, the conventional process for bipolar transistors having a buried region is by doping the substrate, via ion implant, prior to epitaxial growth of the collector region. However, since epitaxial growth is a relatively high temperature step, which takes significant time, the dopant implanted for the buried region will diffuse in all directions from its implanted location. The additional vertical diffusion will tend to decrease the collector-to-base breakdown voltage of the bipolar transistor, and the lateral diffusion of the buried region from the epitaxial growth will increase the required spacing between the location of the subcollector and circuitry which is to have a separate subcollector, or which is not to have a subcollector. Both of these effects limit the scalability of the transistors.
Integrated circuits which have bipolar transistors and MOS transistors of both channel conductivity types formed into the same substrate may have both n-type and p-type buried regions therein, such as described in copending applications Ser. Nos. 129,261 and 129,271, both filed Dec. 7, 1987 and assigned to Texas Instruments Incorporated. Accordingly, the lateral diffusion of the buried regions of both conductivity types during epitaxy requires increased spacing therebetween as compared to the spacing for a single subcollector device. The diffusion which is used to form the well regions in the epitaxial layers over the buried regions will also further diffuse the buried regions, further increasing the required spacing.
It is an object of this invention to provide a bipolar transistor having a high conductivity surface contact to a buried subcollector.
It is another object of this invention to provide such a transistor which has its subcollector formed subsequent to the formation of the epitaxial layers.
It is another object of this invention to provide such a transistor which does not have a subcollector region directly under its extrinsic base diffusion.
It is another object of this invention to provide such a transistor which has its subcollector formed by a process compatible with the formation of MOS transistors on the same substrate.
Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification in conjunction with the drawings.